Add notice about Ruby default changing to 2.4 to UPDATING

[freebsdcodereview_zofrex.com]

Summary

The Ruby default version was changed from 2.3 to 2.4 on 2017/12/24 but UPDATING
was not updated. I'm not 100% sure what the contents should be, I have just
copy pasted the instructions from last time and changed the numbers. The
portmaster instructions seem to have worked for me.

I'm not sure what the date should be (today? or the date it changed?) nor the
author (me or the person who made the original change?).

[freebsdcodereview_zofrex.com]

Oops, editor changed whitespace throughout the file. Fixed.

[freebsdcodereview_zofrex.com]

Adding swills, who I think is the person that changed the default Ruby version?